Igneous rocks are classified by using grainsize, silica content, and/or silica saturation. Other methods of classification include the International Union of Geological Sciences (IUGS) scheme and the Total Alkalies vs Silica scheme.

Grain Size

Based on the predominant grainsize that reflects the depth at which molten rocks form within the Earth. Three types of rock can be identified:

Volcanic rocks solidify close to the Earth’s surface. Because they cool quickly they have a finer-grained matrix (called ground mass). They may contain some larger crystals that formed earlier further down (called phenocrysts).

Plutonic rocks form deeper within the Earth and the slower cooling allows them to crystallise as coarse-grained rocks.

Hypabyssal or subvolcanic rocks form at intermediate depths (generally as dykes and sills) and so tend to be medium-grained.

Silica content

Silica content (SiO2) which also controls the minerals that crystallise is used to further classify igneous rocks as follows:

Ultrabasic: usually less than 45% silica (mostly dark minerals such as olivine and pyroxene), e.g. peridotite.

Silica saturation

This is similar to modal composition, but allows distinction between feldspathoid-bearing and feldspathoid-free rocks.

Rocks that are undersaturated in silica will crystallise silica-poor minerals like feldspathoids or olivine.

Those that are just saturated in silica will lack feldspathoids or quartz.

Those that are oversaturated in silica will crystallise quartz among the minerals.

International classification

The International Union of Geological Sciences (IUGS) classification scheme is the standard scheme for igneous rocks and uses the minerals present. It is in the form of triangular diagrams where each apex of the triangle represents 100% of a mineral. It splits into feldspathoid-bearing (i.e. nepheline, sodalite) and feldspathoid-free rocks. Then, it takes the relative percentages of quartz (Q), plagioclase (P) and alkali feldspars (A). The value of Q, P, and A determines a given field for each rocktype. Similar schemes also classify ultramafic rocks (but using olivine, orthopyroxene and clinopyroxene) and gabbroic rocks.

TAS classification

The TAS (Total Alkalies vs Silica) classification scheme uses chemistry to classify volcanic rocks. The analysis is recalculated to 100% after removing H2O and CO2. It is a simple X-Y graph with the X-axis being SiO2 Wt% and the Y-axis being Na2O+K2O Wt%.

Ultrabasic

Komatiite: these rocks are high in magnesium and crystallised at high temperatures. They commonly display a spinifex texture consisting of intergrown skeletal and bladed olivine and pyroxene crystals in a glassy groundmass. Most are Archean in age.

Other volcanic rock types

Bomb: a volcanic projectile with an average diameter greater than 64 mm and whose shape or surface indicates that it was partially or completely molten during its formation and transport through the air.

Ignimbrite: a pyroclastic tuff consisting of crystal and rock fragments in a matrix of glass shards that are usually welded together.

Obsidian: a volcanic glass that is commonly dark in colour and glassy, with a water content less than 1%.

Perlite: volcanic glass that exhibits numerous concentric cracks. Some are high in water and expand when heated.

Lamproite: a volcanic or subvolcanic rocks that are rich in potassium and magnesium, composed of unusual rare minerals such as K-Ti-richterite, priderite, wadeite, jeppeite, Fe-orthoclase and leucite. The hostrock for diamonds in the AK1 pipe of the Argyle diamond mine, Western Australia.

Lamprophyre: a name for a group of subvolcanic rocks that are strongly porphyritic in mafic minerals such as biotite, amphiboles and pyroxenes in a groundmass of feldspar. They commonly occur as dykes.